1. Field of the Invention
This invention relates to a continuous process for the extraction and esterification of carboxylic acids. More specifically, this invention relates to a process for the extraction of a first carboxylic acid, denoted RCO.sub.2 H, from an aqueous stream into an organic stream. This organic stream may comprise a carboxylic acid ester reactant, denoted R"CO.sub.2 R'. The ester reactant, R"CO.sub.2 R', and the first acid, RCO.sub.2 H, are contacted with a catalyst to form a product carboxylic acid ester, denoted RCO.sub.2 R', and a second carboxylic acid, denoted R"CO.sub.2 H. The reactant ester, R"CO.sub.2 R', is regenerated by esterifying the second acid, R"CO.sub.2 H. The reactant ester, R"CO.sub.2 R', is then recycled to the process. In this process R is aliphatic or aromatic, R' is alkyl and R" is aromatic or aliphatic having at least about 4 carbon atoms. Where the ester reactant is a bifunctional polymer, having both sulfonic acid and R' carboxylic acid ester pendant functional groups, this process can be performed in the absence of a catalyst.
2. Description of the Prior Art
Carboxylic acid esters have many diverse uses. Many aromatic and alkyl esters are useful in perfumes and artificial flavorings. For example, methyl butyrate and ethyl butyrate are used in the formation of artificial rums. Some carboxylic acid esters are also used as solvents in varnishes, lacquers and paints. Some carboxylic acid esters, particularly those formed from unsaturated acids, may be reacted further to form polymers useful in the manufacture of fibers and coatings or films.
Conventionally, esters can be formed by either a batch or a continuous process. While a continuous process is preferred for commercial operations, it is often impracticable due to problems in product separation.
Carboxylic acid esters can be formed using one of several reaction pathways. One pathway is ester alcoholysis, depicted below: ##STR1##
The reactant ester, RCO.sub.2 R", is reacted with an alcohol, R'OH, usually in the presence of a catalyst. The reaction results in the transfer of the R' group to the ester, followed by cleavage of the R" group to yield a product ester of the general formula RCO.sub.2 R' and an alcohol, R"OH.
A second possible pathway involves the reaction of an acid with an ester. This reaction, called "ester acidolysis", can be represented as follows: ##STR2##
In this reaction R' is transferred from the reactant ester, R"CO.sub.2 R', to the first acid, RCO.sub.2 H. This results in a product ester of the general formula RCO.sub.2 R' and a second acid, R"CO.sub.2 H.
Esters may also be formed by the addition of an alcohol to an organic acid. This process, depicted below in Reaction III, results in the transfer of the alcohol to the organic acid with H.sub.2 O as a by-product: ##STR3##
In many known commercial operations for producing carboxylic acids the product is recovered from the reactor in a dilute aqueous solution. Since most esterification reactions require that the reactants be in concentrated form, extra steps are required to concentrate the carboxylic acid from a product stream before the acid can be introduced to an esterification process. These extra steps, such as distillation, are often expensive, may result in product losses and impede a smooth coupling of the esterification process with the process producing the carboxylic acid.
Esterification reactions usually involve a reversible equilibrium which can be driven forward by removal of one or more of the products. In a continuous process, product removal is frequently accomplished by product distillation. Alcoholysis (Reaction I), acidolysis (Reaction II) or addition of alcohol to an organic acid (Reaction III), however, frequently give rise to azeotropic mixtures. For example, when acrylic acid is esterified by ethanol according to reaction III the reaction products are ethylacrylate and water. A ternary azeotrope is formed by the combination of ethanol, ethylacrylate and water. Therefore, when distillation is executed an extra step must be performed to separate ethylacrylate from the azeotrope. This makes the process more time consuming and expensive than if the ester could be distilled as an isolated product. Also, when an azeotrope is formed involving a product and a reactant and the azeotrope is continuously removed, the effect of product removal on the position of the equilibrium is at least partially negated by the simultaneous removal of one of the reactants.
U.S. Pat. No. 4,280,009 to Erpenbach discloses a continuous process for the production of 2-ethylhexylacrylate free from dioctylether. U.S. Pat. No. 3,354,199 to Lachowicz discloses a process for the production of ethylacrylate by reacting acrylic acid with ethanol in the presence of a catalyst and entrainer to produce ethylacrylate and water.
U.S. Pat. No. 4,280,010 to Erpenbach discloses a continuous process for the production of alkylacrylates free of ether by the reaction of acrylic acid or methacrylic acid with an alcohol in the presence of a catalyst to form an alkylacrylate and water.
U.S. Pat. No. 4,117,238 to Ackermann involves a process for the transesterification of acrylic and methacrylic esters according to reaction I wherein acrylic or methacrylic esters are reacted with alcohols in the presence of an alkali metal cyanide. U.S. Pat. No. 4,228,084, also to Ackermann, involves a similar process for the production of glycidylmethacrylate from methylmethacrylate and glycidol.
U.S. Pat. No. 4,074,062 to Murakami discloses a process for producing unsaturated carboxylic acid esters by reacting unsaturated carboxylic acids with alcohols in the presence of a catalyst. U.S. Pat. No. 4,202,990 to Murakami involves a process for producing unsaturated carboxylic acid esters by following the reaction steps for Murakami U.S. Pat. No. 4,074,062 using a catalyst consisting of chelate compounds of zirconium and/or calcium instead of the barium, thallium or molybdenum compounds of U.S. Pat. No. 4,074,062.
U.S. Pat. Nos. 3,293,283 to Dobson, 3,328,439 to Hamilton, 3,714,234 to White, 3,784,537 to Fields, 4,112,235 to Schmerling, and 4,205,182 to Izumi disclose esterification processes similar to those described above.